This invention relates to a Magnetic Resonance Imaging (MRI) apparatus. More particularly, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field that makes nuclei, such as hydrogen atomic nuclei, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatial magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during the imaging pulse sequence. The radiofrequency coil(s) creates an excitation frequency pulse that temporarily creates an oscillating transverse magnetization that is detected by the radiofrequency coil and used by the computer to create the image.
Generally, very high field strength is characterized as greater than 2 Tesla (2 T). In recent years, there has been an increase in usage of MRI systems at field strengths above the typical 1.5 Tesla. Research systems have been built as high as 8 Tesla. Systems are now commercially available at 3 Tesla and 4 Tesla. The systems are primarily used for research in function MRI (fMRI) and human head related imaging and spectroscopy studies. Higher magnetic field strength imposes challenges on the RF coil, such as balancing inductance and capacitance at higher frequencies (greater than the typical 64 MHz). At very high magnetic fields, and therefore very high Larmor frequencies, standard birdcage coils with moderately narrow rung copper strips will have relatively high inductance requiring very low capacitor values in order to resonate the coil. This is problematic for a number of reasons. Firstly, high currents through small value capacitors will have very high voltage potential across them which can then have a local stray electric field that can dissipate RF power in the form of heat in an imaging subject.
The U.S. Food and Drug Administration (FDA) has imposed limits, referred to as Specific Absorption Rate (SAR), on the level of electromagnetic energy which can be absorbed by a patient or medical personnel during MRI scanning. These limits help reduce the risk of RF induced burn on the patient, or imaging subject. There are two types of electric fields associated with MRI. The first is due to time-varying B1 magnetic field present within the imaging subject and the second type is due to electric charges on the capacitors in the RF coil structure.
What is needed is a RF coil assembly for MR imaging at high magnetic field strengths and reduced electromagnetic energy exposure to the imaging subject.
A radio frequency (RF) coil assembly for a very high field Magnetic Resonance Imaging (MRI) system is provided. The RF coil assembly comprises a plurality of conductors arranged cylindrically and disposed about a patient bore tube of the MRI system. Each of the conductors is configured for the RF coil assembly to resonate at substantially high frequencies. Further, the RF coil assembly comprises a plurality of capacitive elements disposed between and connecting respective ends of the conductors and further disposed in a spaced-apart relationship with the patient bore tube. The capacitive elements are for electrically interconnecting the plurality of conductors at the respective ends of the conductors.